Nature determines when things get underway
What happens inside a debris flow? When does a stream transport rock and debris? Large-scale experiments in mountain streams provide answers.
The noise is deafening. A passing mountain biker stops and looks around in bewilderment. A sign under the flashing warning light warns: “Danger! Debris flows can occur at any time – even when the weather is good.” Only when WSL technician François Dufour calls out: “Don’t worry, it’s only a test”, does the biker continue riding along the almost completely dried up stream bed. No one thinks that the trickle between gravel and debris, in Illgraben near Leuk (Valais), will cause any harm. However, it can turn into a deadly wave of mud and water in a matter of seconds. When it rains, sometimes combined with snowmelt, hundreds of tons of mud, gravel and boulders thunder down into the valley as a debris flow. This sort of debris flow is more powerful than a flood with sediment and can cause a correspondingly high amount of damage. Thanks to the geological structure of Illgraben, several of these events occur each year, which makes it an ideal place to study debris flows.
Since May 2000, WSL has had a number of measurement instruments installed in Illgraben, including a geophone that measures seismic ground motion. The geophone records the vibrations that signal the beginning of a debris flow and automatically starts video recording. Radar and lasers record the flow height and speed of the mud and debris mass in several locations. At the outlet of the torrent channel, where the Illgraben flows into the Rhone, sensors in a side wall measure the forces inside the debris flow mixture. And underneath the bridge that carries the cantonal road, a scale records the weight of the material flowing past.
“This location provides us with a unique opportunity to research the inner workings of debris flows: their composition, their flow behavior, their characteristics,” explains technician Dufour. The data allows researchers to improve their understanding of the processes that occur during a debris flow and optimize computer simulations. This helps engineers and planners prepare hazard maps and assess any protective measures.
Dufour is an engineer and one of the technicians who also maintains the warning system in Illgraben, in addition to the scientific measurement devices. Inspections and maintenance work are required in particular after major debris flows. He has to take a different route to reach the measurement devices each time, as the dirt path is gradually falling victim to erosion. Today, he has to crawl over the rocky ground on all fours, using bushes and branches to lift himself up in order to reach the highest instrument.
WSL installed the warning system in 2007 on behalf of the municipality of Leuk/Susten. As soon as the warning devices register a debris flow, local security officials receive notification automatically via SMS. At the same time, visual and audible alarms signal the danger at three stream crossings. The findings of scientific observations and an emergency plan developed in conjunction with the municipality serve as the basis for the early warning system. In addition, in order to be able to detect a major debris flow in good time, rainfall in the catchment area is measured automatically and field surveys are carried out on a regular basis – by helicopter in extreme situations. In addition to its protective function, the warning system also serves as a pilot installation for the development of warning measures in other locations.
WSL also measures how much material may be carried in a stream in Alptal (Schwyz), albeit for a different reason. Here, it rains often and heavily, and as the soil is clay-rich, the water does not drain well. So the risk of flooding is high. Although this may pose a risk to the surrounding communities, it is a boon to researchers: The area has the ideal conditions in which to explore the factors that contribute to flooding. In the 1960s, WSL equipped various mountain torrents in the Alptal with measurement devices to find out how the forest affects the formation of floods and water quality. Researchers have also been looking into how sediment and driftwood are transported, among other issues, since the 1980s and have developed models for forecasting floods for several years.
Dieter Rickenmann, a researcher in the mountain torrents group, stands with two Japanese colleagues looking at the Erlenbach stream in Alptal. Despite the sun, it is cool, the air is damp and their fingers quickly become clammy. When they reach the large retention basin, Rickenmann shows his guests how WSL researches sediment transport in one of the most active mountain torrents in Switzerland. There is a reason the visitors have traveled so far: The WSL’s sediment measurement equipment is not found anywhere else in the world.
WSL workers developed a method to monitor and measure the sediment indirectly. The geophone records the vibrations that sediment particles cause when they roll over steel plates. The steel plates are embedded in the streambed, with the geophone sensors mounted on their underside. The signals are calibrated using direct measurements of the transported sediment. To do this, the researchers attached collecting baskets at the inlet of the retention basin. These baskets are mounted on horizontal rails and are pulled into the flow of water automatically by winches whenever the flow is strong. There, they collect sand, gravel, and cobbles transported along the stream bed. The material collected in the retention basin is also used to calibrate the measurements of the geophone, by measuring the deposited sediment.
“Using these measurements, we were able improve the calculation methods for sediment transport significantly and develop a computer simulation model. We can now use this model to calculate how much sediment will be transported under what conditions – even for longer stretches of mountain streams,” explains Rickenmann. The model was used as part of National Research Program 61 ‘Sustainable Water Management’. Researchers looked at how climate change might affect sediment transport and the living conditions for the brown trout.
The collecting baskets remain motionless today, as there is hardly any water in the Erlenbach, let alone sediment transport. The fact that the mountain stream can look quite different is documented by the photos from the last major flood in June 2007. Following a rainstorm, the Erlenbach filled the inlet basin with stones, mud and driftwood in about two hours. “This sort of extreme event takes place here about every 10 years. We have about 15 to 20 floods with sediment transport annually,” says Rickenmann. Fortunately, the stream did not cause any damage to the nearby hamlet of Brunni.
Planning, construction and maintenance of a system such as at the Erlenbach is expensive. The inlet basin must be dredged regularly to ensure that it can collect a good amount of sediment at all times, even in a major flood. The excavator loads some 1,000 cubic meters of material in 85 trucks, which then bring it to a waste disposal site. “The maintenance is intensive, especially for the flow measurements that we need to calculate the sediment transport. A staff member inspects the measurement data each week and cleans the dirty instruments,” explains Rickenmann.
Perhaps in the future it will be possible to calibrate the geophones in the lab, which would be easier and cheaper. Doctoral student Carlos Wyss looked into this in his doctoral work at WSL. His results were encouraging, although the laboratory calibrated methods were not quite as precise as had been hoped.
Hot, dry Illgraben and wet, cool Erlenbach: The commonality in both of WSL’s large-scale experimental facilities is that the experiments are triggered by nature, rather than man. Rickenmann explains: “We observe natural processes that absolutely could not be triggered artificially on this scale.” (Gottardo Pestalozzi, Lisa Bose, Diagonal 1/16)